Reports say that two US congressmen have written to Apple asking why the firm allows the practice on its iPhone, as it contravenes app developer guidelines.

Twitter spokeswoman Carolyn Penner told the BBC and Los Angeles Times: "We want to be clear and transparent in our communications with users.

"Along those lines, in our next app updates, which are coming soon, we are updating the language associated with Find Friends to be more explicit."

She also said that in place of "scan your contacts", the site will use "upload your contacts" and "import your contacts" instead, for iPhone and Android apps respectively.

A spokesman for Apple said: "Apps that collect or transmit a user's contact data without their prior permission are in violation of our guidelines.

"We're working to make this even better for our customers, and as we have done with location services, any app wishing to access contact data will require explicit user approval in a future software release."

The copying of address book contacts was made evident when an app developer in Singapore noticed that his contacts had been copied from his iPhone without his consent by a social network called Path.